[DOCs][RTD] Adding sphinx-rtd-theme dependency

.readthedocs.yaml

@@ -31,6 +31,6 @@ sphinx:
 # Optional but recommended, declare the Python requirements required
 # to build your documentation
 # See https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
-# python:
-#    install:
-#    - requirements: docs/requirements.txt
+python:
+  install:
+    - requirements: docs/source/requirements.txt

docs/source/requirements.txt

@@ -0,0 +1 @@
+sphinx-rtd-theme